The present invention relates in general to a locking device for a drive point assembly and, more particularly, to a method and apparatus for releasably locking the drive point at the forward end of an apparatus such as a soil sampling probe.
For many years, soil samples have been obtained to determine soil conditions prior to construction of structures on the ground and to locate certain mineral deposits. Furthermore, these samples may be used for studying chemical dissipation and residue, for determining the concentration of environmental contaminants, for investigating hazardous waste sites and for other uses well-known in the art. The fundamental requirements for a soil sampling probe include the ability to penetrate the soil to a desired depth, to obtain a sample of soil at the desired depth and to remove the probe from the ground with the sample intact. To this end, it is desirable to provide a durable drive point at the forward or leading end of a soil sampling probe to penetrate the soil. The point must be firmly coupled to the probe to be able to withstand the forces associated with static or percussion driving techniques. Importantly, the drive point must be capable of being released from the forward end of the probe at any desired depth so that a soil sample may be taken into the sample chamber of the probe.
A prior art soil sample probe is described in U.S. Pat. No. 5,186,263 to Kejr et al., which is owned by the assignee of the present invention and is hereby incorporated by reference. The drive point assembly disclosed in the '263 patent utilizes a piston stop and an elongated piston rod to maintain the drive point at the forward end of the sample tube as the probe is driven into the ground. This prior art construction presents several design problems. First, the piston rod must be about the same length as the sample tube, which is typically four feet long. As the probe is driven into the ground, the relatively slender piston rod must absorb the resistive force encountered by the point and is therefore subject to buckling. Likewise, the strength of the connection between the piston stop and the drive head must be sufficient to withstand the force imparted to it from the piston rod. Moreover, a conventional drive point presents a shoulder which abuts a mating shoulder at the forward end of the sample tube and which may be exposed to shearing forces. Such a construction is relatively expensive and is inherently subject to the possibility of damage during use.
Another problem associated with prior art drive points such as the assembly disclosed in the '263 patent involves the process of extracting the probe from the ground. Typically, a plurality of probe extensions are connected to the sample tube as the probe is driven into the ground, and the specific number of probe extensions added will be dictated by the desired sample depth. Preferably, a pull cap is used to extract the probe from the ground, one section at a time. However, when the soil sample which is taken into the sample tube forces the drive point to the top of the sample tube, the piston rod connected to the drive point protrudes upwardly through the drive head and several feet into the adjoining probe extension. Consequently, after the last probe extension has been removed, the pull cap cannot be placed over the sample tube due to the protruding piston rod. Therefore, some additional equipment, which is otherwise unnecessary, must be used to remove the sample tube from the ground.